Silicone rubbers, on account of their heat resistance, cold resistance, safety, good appearance (transparency), good feel and good durability, are widely used as materials for molding toys, tableware, toothbrushes and the like for infants and small children, and particularly baby bottle nipples and pacifiers. Addition reaction-curing type silicone rubber compositions are especially preferred for use in the above applications on account of the fact that, unlike organic peroxide curing-type silicone rubber compositions, they do not generate by-products due to the decomposition of organic peroxides, and especially on account of their safety.
Silicone rubber nipples for baby bottles that are currently in wide use have a durometer A hardness of from 30 to 50, but silicone rubber nipples of even lower hardness are desired, either in order to diversify the design or for infants having a weak ability to suckle. In terms of applications for low-hardness silicone rubber, there are cases in which low-hardness silicone rubber is used in the portion of a mask that comes into contact with the face. When a low-hardness rubber is used as a mask material, the discomfort experienced on wearing the mask can be reduced. However, lowering the amount of reinforcing silica in silicone rubber so as to produce a low-hardness product leads to a loss of strength, as a result of which cracks form in the rubber when the molded article is removed from the mold, and the rubber is prone to being cut during use. On the other hand, if one tries to achieve a low hardness without reducing the amount of reinforcing silica, that is, by disrupting the addition crosslinking balance, a rubber having permanent set or a rubber that exhibits surface tack is obtained; hence, the rubber has a feel to the touch that makes it ill-suited for use in baby bottle nipples or as a mask material.
In order to resolve such problems, a method has been disclosed which adds a raw silicone rubber to a liquid silicone rubber composition, thereby lowering the rubber hardness and eliminating the sticky feeling (Patent Document 1: JP-A 2003-321609). However, when a raw silicone rubber is added to a liquid silicone rubber composition, even if no crosslinking ingredients are present, the viscosity rises markedly over time, leading to problems with storage stability.